Transistor amplifiers are inherently rich in harmonics. This is because of the non-linear junction that all transistors have. If you are using simple LPF, not diplexer.
In contrast, this design passes the lower desired signal to the antenna port using a low-pass filter, and sends the higher frequency by-products via a. High-pass filter into a termination. The diplexer filter presents the constant load impedance to power amplifier very close to 50 Ohm from dc to. Beyond 50 MHz, providing high stability and linearity.

You may read the article by W0IYH in QEX 1999/07 magazine. This LPF diplexer is good for any kind of 300W max power amplifiers on the market. The diplexer consists of 6 relay-switched units and covers 1.8 to 54 MHz amateur radio bands. It has 1 input port and two outputs, one for antenna and one for RX input terminal of your radio.

It has SWR bridge onboard, which can be used for SWR measurement or protection. The cores are T68 and T50 by Micrometals, relays are HJR1-2CL 12 VDC, capacitors are low ESR multylayer ceramic SMD high voltage 3.15 KV and mica units on 160 meters , connected in parallel to meet the high power requirements. The diplexer terminal load is 12pcs 2W each resistors. FR4 double-sided PCB size 165 x 130 x 20(H) mm. The filters are manually tuned on each band to get the SWR as low as 1.2:1.
The item "LPF diplexer 1.8-54MHz 300W SSB/CW HF amplifier low pass filter MOSFET LDMOS RM" is in sale since Thursday, March 16, 2017. This item is in the category "Consumer Electronics\Radio Communication\Ham, Amateur Radio\Ham Radio Amplifiers". The seller is "r3kbo_2" and is located in Voronezh, default. This item can be shipped worldwide.ru